Enterococci are the 3rd most common cause of endocarditis, behind streptococci and staphylococci, and the 2nd or 3rd most common cause of hospital acquired infections, with Enterococcus faecalis being the predominant species isolated. Antimicrobial resistance likely facilitates the establishment of enterococci in nosocomial infections and certainly makes it more difficult to successfully treat patients, particularly those with endocarditis. The central hypothesis of this project is that by better understanding enterococci, new therapeutic or preventative modalities can be developed. Work during a previously funded grant identified and characterized a number of antigen encoding genes; a polysaccharide gene cluster (epa) that appears to influence virulence in mice; different adherence phenotypes, and a gene, ace, that appears to be involved in adherence; and a gene locus with homology to the accessory gene regulator (agr) locus of staphylococci that is involved in expression of an E. faecalis gelatinase and a serine protease that also influence virulence in mice. In this application, the investigators propose (1) to verify that the E. faecalis agr-like locus regulates gelE and sprE and determine if all are important for virulence; to investigate the distribution of these genes among E. faecalis; and to determine how the enterococcal agr-like locus is regulated and if it, like the staphylococcal agr, regulates other genes. They also plan (2) to test the hypothesis that Ace (a newly described adhesin for collagen of enterococci) is the cause of the adherence we have reported and is important for virulence; to explore the regulation of Ace production; and to determine the distribution and effect of variations in ace, if Ace elicits an antibody response in humans (using recombinant Ace and patient sera) infected by E. faecalis and if antibody made during infection, or antibody to recombinant Ace, is protective. In their third specific aim, they plan (3) to establish if the polysaccharide gene cluster is the cause of a recently described mucoid phenotype, to study its regulation, and to further test its contribution to adherence to foreign material, virulence and protection. They will also explore a system for constructing non-polar deletion mutants using counter-selection based on their prior work with the E. faecalis pyr genes, and to explore additional assays that would help to avoid lethality models. The investigators hope that results from this work will provide solid leads in the quest for methods to prevent, control, or combat E. faecalis infections.